Oil country products are often subject to demanding conditions. In particular sour gas wells and certain oil fields certain highly corrosive actors and when combined with the elevated temperatures present wreak havoc with metallic members.
Accordingly, nickel-base alloys have been repeatedly selected for these demanding applications.
For example, INCO.RTM. alloys G-3 and C-276 and INCOLOY.RTM. alloy 825 (INCO and INCOLOY are trademarks of assignee) have been specified for use in deep sour gas wells and also for seamless pipes and liners in oil fields. For these applications the materials must meet stringent specifications dictating the acceptable range of room temperature tensile properties, hardness, macrostructure, microstructure and corrosion properties. Of particular interest to the energy companies is the room temperature 0.2% yield strength which is usually restricted to narrow ranges (i.e. 758.4-896.3 MPa [110-130 ksi], 861.9-999.7 MPa [125-145 ksi], 896.3-1034 MPa [130-150 ksi]).
INCO alloy G-3 is a nickel-chromium-iron alloy with additions of molybdenum and copper. It has good weldability and resistance to intergranular corrosion in the welded condition. The low carbon content helps prevent sensitization and consequent intergranular corrosion of weld heat-affected zones. It is most useful in corrosive environments. The nominal composition of alloy G-3 is about 21-23.5% chromium, 18-21% iron, 6-8% molybdenum, up to 5% cobalt, 1.5-2.5% copper, up to 1.5% tungsten, up to 1% silicon, up to 1% manganese, balance nickel, and traces of other elements.
INCO alloy C-276 is a nickel-molybdenum-chromium alloy with an addition of tungsten having excellent corrosion resistance in a wide range of severe environments. The molybdenum content makes the alloy especially resistant to pitting and crevice corrosion. The low carbon content minimizes carbide precipitation during welding to maintain corrosion resistance in as-welded structures. The nominal composition is about 15-17% molybdenum, 14.5-16.5% chromium, 4-7% iron, 3-4.5% tungsten, up to 2.5% cobalt, up to 1.0% manganese, balance nickel, and traces of other elements.
INCOLOY alloy 825 is a nickel-iron-chromium alloy with additions of molybdenum and copper. It has excellent resistance to both reducing and oxidizing acids, to stress corrosion cracking and to localized attack such as pitting and crevice corrosion. The nominal composition is about 19.5-23.5% chromium, 38-46% nickel, 2.5-3.5% molybdenum, 1.5-3% copper, 0.6-1.2% titanium, up to 1% manganese, at least 22% iron and traces of other elements.
Alloy 825, having an appreciable quantity of iron, has been heat treated by assignee in the past to strengthen tubes. By inserting the finally reduced tube into a salt bath having a temperature of about 482.degree. C. (900.degree. F.) for about one half hour, the resultant room temperature yield strength and tensile strength improved, on average about 5% and 7% respectively given an initial 150 ksi (1034.1 MPa) tensile strength and 130 ksi (896.2 MPa) yield strength.
There are differences in alloy G-3 and alloy 825 that do not permit straight expected comparisons. Besides different chemistries, alloy 825 forms a M.sub.23 C.sub.6 phase, whereas alloy G-3 forms a (Ni,CR,FE,CO).sub.3 (Mo.sub.1 W).sub.2 (mu) phase. These phase and chemistry differences result in different corrosion and work hardening behaviors.
A typical processing route for the manufacture of oil and gas field pipe is to produce a billet, extrude the billet to a tube, anneal the tube, reduce the tube, solution anneal the tube and subject the tube to a final tube reduction. The final tube reduction is performed with a controlled level of cold work to attain the desired yield strength. See FIG. 1 (solid lines). Unfortunately, for the alloys a prohibitively high level of cold work is necesary to reach the desired high yield strength levels. To overcome this limitation the annealing temperature can be reduced as the material's strength will increase as the anneal temperature decreases at a fixed level of cold work. However, this practice is limited by: (1) the precipitation of undesirable phases formed at lower temperatures; (2) the reduction of the material's corrosion resistance; and (3), in some cases, the reduction of room temperature ductility. Hence, it is desirable to define a processing method to increase the material's strength without sacrificing the other properties (i.e., corrosion resistance).